A DPM filter is used to trap diesel particulate matter RPM) contained in exhaust gas as a measure against the black smoke that is produced by diesel engines. In an engine comprising a DPM filter, the temperature of the exhaust gas is raised at a timing when the DPM has accumulated to a certain degree, as a result of which the accumulated DPM is burned, thus regenerating the DPM filter.
If the amount of trapped DPM is large when the DPM filter is to be regenerated, the amount of heat generated by combustion of the DPM increases, causing the temperature of the DPM filter to rise excessively in correlation with the exhaust gas temperature, which is raised to promote regeneration. As a result, the durability of the carrier decreases, leading to thermal degradation of the catalyst in a catalyst-carrying DPM filter, and thus the DPM filter regeneration performance and other catalytic actions may deteriorate. To avoid this problem, the amount of trapped DPM must be estimated accurately, and regeneration must be performed before the trapped DPM amount becomes excessive.
JP2002-256846A, published by the Japan Patent Office in 2002, discloses a method for determining the regeneration timing by estimating the trapped DPM amount from a map using the exhaust gas flow, the difference between the inlet pressure and outlet pressure of the DPM filter (hereinafter referred to as the DPM filter differential pressure), or another index expressing a similar exhaust gas resistance value.